This invention relates to multi-media communications, and specifically to a method and apparatus to limit microwave interference in unlicensed RF devices.
Bluetooth (BT) is a short-range communications specification, designed for operation in the 2.4 GHz microwave frequency industrial, scientific and medical (ISM) band. Bluetooth(trademark) is a trademark/service mark of Telefonaktiebolaget LM Ericsson, of Stockholm, Sweden for telecommunication equipment, computer communication equipment, including radio modems; and telecommunication and computer communication services. BT is intended primarily as a replacement for cables, providing connectivity over a short range between, for example, a PDA and Laptop or a Laptop and Cellular Phone. Bluetooth is also an RF alternative to IrDA connectivity. BT devices operate in accordance with the FCC Part 15 rules for unlicensed operation. It must share this band with a number of other communications devices, also regulated by Part 15, e.g., 802.11 W-LAN, as well as consumer microwave devices, regulated by FCC rules Part 18, e.g., microwave ovens and RF lighting.
Devices regulated by Part 18, such as microwave ovens, are considered primary users of the ISM band. Communications devices, regulated by Part 15, are considered secondary users and are required to have a low probability of creating interference and be sufficiently robust to avoid malfunctioning as a result of interference from other ISM band devices, which operate under either Part 15 or Part 18. For these reasons, Part 15 rules require the use of spread spectrum techniques, either frequency hopping or direct sequence. The BT specification is no exception, and has adopted a frequency-hopping scheme. BT hops in a pseudo-random manner among the 23 or 79 different channel frequencies in the ISM band, at a rate of 1600 hops per second.
If another ISM band device, e.g., a communications system or oven, is operating in close proximity to a BT device, high interference will be present in some portion of the ISM band. Thus, some number of the 23 or 79 channel frequencies available to BT will be unusable for reliable data transfer. However, because BT hops in a pseudo-random manner among these channel frequencies, a BT device will spend only a small portion of its time on any bad channel frequency and will continue to transmit data on the unaffected channels.
The net result of this interference on BT channels is that a BT device will continue to operate, but will operate at reduced performance, e.g., with increased delay, degraded voice or video quality, or reduced data throughput. This reduction in throughput, or Quality of Service (QoS), may be substantial and noticeable to an end user of a BT device.
Prior art in the unlicensed band avoids interference problems by simply hopping in a random manner. While this allows a device to continue communicating, it does not provide a means to mitigate the resulting throughput and QoS reduction. Other systems employ similar techniques in which the channel frequency usage pattern is directly modified to avoid bad channels.
U.S. Pat. No. 4,479,215 to Baker, granted Oct. 23, 1984, for Power-line carrier communications system with interference avoidance capability, describes a system which uses two channels in a power-line communications system to allow a switch in channel frequency if the current channel frequency is experiencing bad interference.
U.S. Pat. No. 5,444,696 to Petranovich, granted Aug. 22, 1995, for Frame structure using consecutive slot assignments for mobile communications, provide a means to provide a TDMA mobile station more time to scan alternative frequencies in order to perform more effective cellular handover.
U.S. Pat. No. 5,574,979 to West, granted Nov. 12, 1996, for Periodic interference avoidance in a wireless radio frequency communication system, describes a signal processing techniques to mitigate problems caused by a periodic interference source, i.e., one which pulses interference on a regular time basis.
U.S. Pat. No. 5,734,678, granted Aug. 12, 1997, U.S. Pat. No. 5,687,194, granted Nov. 11, 1997, and U.S. Pat. No. 5,657,358, granted Mar. 31, 1998, all to Paneth et al., and all entitled Subscriber RF telephone-system for providing multiple speech and/or data signals simultaneously over either a single or a plurality of RF channels, describe a robust communications system design which does not employ interference avoidance techniques.
U.S. Pat. No. 5,710,762 to Petranovich, granted Jan. 20, 1998, for Frame structure having non-symmetrical slot assignments for mobile communications, describes the provision of a TDMA mobile station which is given more time to scan alternative frequencies in order to perform more effective cellular handover.
U.S. Pat. No. 5,754,947, granted May 19, 1998, and U.S. Pat. No. 5,884,171, granted Mar. 16, 1999, both to Tanabe et al., and both for Radio communication apparatus and method for preventing hidden terminals from interrupting communications, are related to avoiding the classic hidden terminal interference problem of wireless communications. This is essentially a technique to avoid starting transmission on an interfered channel.
U.S. Pat. No. 5,784,402 granted Jul. 21, 1998, to Feher, for FMOD transceivers including continuous and burst operated TDMA, FDMA, spread spectrum CDMA, WCDMA and CSMA, describes an advanced modulation technique.
U.S. Pat. No. 5,852,604 to Cooley et al., granted Dec. 22, 1998, for Modularly clustered radiotelephone system, describes an improved power saving technique using synthesizer techniques.
U.S. Pat. No. 5,875,180 to Wiedeman et al., granted Feb. 23, 1999, for Satellite telephone interference avoidance system, describes use of techniques to avoid interference problems in a geographical subset of a satellite""s communications footprint due to location of terrestrial systems using the same frequency in these regions. It statically assigns frequencies based on this non-changing interference environment.
A RF communications system having improved RF interference characteristics for use with at least a pair of RF transceivers which communicate using a packet transfer protocol, in a frequency hopping scheme, wherein the packets may be of various lengths, wherein each frequency occupies a frequency channel slot in a RF band, and wherein a subsequent frequency is known, including, in at least one of the transceivers: a detector for detecting distressed channel frequencies having an unacceptable level of interference thereon; a tracker for keeping track of the distressed channel frequencies and of channel frequencies which are not distressed; and a frequency selector for selecting a next transmission frequency from the available non-distressed channel frequencies, including a look-ahead mechanism for determining which channel frequencies will be used in subsequent frequency channel slots, and an interference avoidance mechanism for determining, from said tracker, which of the subsequent frequency channel slots is scheduled to operate at a distressed frequency, and wherein said frequency selector avoids beginning transmissions in any frequency channel slot associated with a distressed channel frequency by adjusting the schedule.
A method of improving RF interference characteristics in a RF communications system having at least a pair of RF transceivers which communicate using a packet transfer protocol, in a frequency hopping scheme, wherein the packets may be of various lengths, wherein each frequency occupies a frequency channel slot in a RF band, and wherein a subsequent frequency is known, including, in at least one of the transceivers: detecting distressed channel frequencies having an unacceptable level of interference thereon; tracking distressed frequencies; and selecting for use by the system only those channel frequencies which are not distressed.
An object of the invention is to provide a packet scheduling scheme employing the long packet types allowed in BT to avoid transmitting on a channel frequency having interference present thereon.
Another object of the invention is to maintain a high data throughput and QoS, despite the presence of a nearby interferer, such as a microwave oven.
A further object of the invention is to provide an improved BT scheme which does not violate any of the requirements of the BT specification.
Another object of the invention is to provide a BT transceiver which is operable with legacy BT devices which do not employ this scheme.
This summary and objectives of the invention are provided to enable quick comprehension of the nature of the invention. A more thorough understanding of the invention may be obtained by reference to the following detailed description of the preferred embodiment of the invention in connection with the drawings.